The present invention relates to charged particle beam equipment which scans a charged particle beam on a specimen to form an image using a signal generated from the specimen by the irradiated charged particle beam.
Examples of apparatus for observing an enlarged image of a specimen using a charged particle beam include a scanning electron microscope, a scanning transmission electron microscope, and a focused ion beam observation processing apparatus. An observation magnification in the charged particle beam equipment is defined by a ratio of the amount of charged particle beam scanned on a specimen surface to an enlarged image generated from detected secondary electrons and the like from a scanned area.
The amount of charged particle beam scanned on a specimen can be arbitrarily varied in accordance with the intensity of an electric field or a magnetic field which is acted on the charged electron beam by a scanning mechanism. For example, for a scanning electron microscope which employs electrons for charged particles and a magnetic field for an electromagnetic lens and electron scanning mechanism, the magnification of a secondary electron beam can be changed by changing the magnitude of a current applied to electron beam scanning coils, and changing an area on a specimen which is scanned by an electron beam.
A narrower area scanned by an electron beam on a specimen results in a larger magnification of a secondary electron image, while a wider scanning area results in a smaller magnification.
FIG. 2 is a diagram illustrating the principles of a mechanism for scanning a charged particle beam. Assume herein that charged particles are electrons. An electron beam 3 moves along an electron beam optical axis 58. Scanning coils 61, 62 are symmetrically disposed on the electron beam optical axis in the X- and Y-directions. The scanning coils are disposed at an upper and a lower stage for applying the electron beam perpendicularly to a specimen. Saw wave scanning signals are applied to the upper scanning coil 59 and lower scanning coil 60, respectively, to cause the electron beam to reach a prefocal position of an objective electromagnetic lens 9 on the optical axis, thereby allowing the electron beam to impinge perpendicularly to the specimen.
The incident electron beam interacts with the specimen to generate secondary electrons 8, specimen forward scattering electrons 12, specimen transmitted electrons 13. Detected signals of these secondary electrons 8, specimen forward scattering electrons 12, and specimen transmitted electrons 13 are synchronized with scanning waveforms to from an enlarged image of the specimen. The magnification of the enlarged image of the specimen depends on the voltages of the scanning waveforms applied to the X- and Y-scanning coils.
FIG. 3B is a graph showing a magnification M at which a specimen is enlarged and a maximum value Vmax of a scanning waveform voltage applied to a scanning coil, where the maximum value Vmax of the scanning waveform voltage has an inversely proportional relationship to the specimen magnification M. While an infinite number of magnifications can be ensured in principle by continuously varying the scanning wave voltage using a variable resistor or the like, the continuously variable magnification is inconvenient and unnecessary in practical use. However, since the observation magnification of charged particle beam equipment generally ranges from a minimum magnification to a maximum magnification which is 103 times higher than the minimum magnification, a reference scanning wave voltage is applied to the scanning coil through voltage attenuators which are provided on a range-by-range basis, as illustrated in FIG. 3A.
The magnification is switched in steps, for example, 1,000 times, 1,500 times, 2,000 times, 3,000 times, . . . , and magnification ranges are defined for ensuring a wide magnification span, such as in the form of a range 1 from 1×103 times to 9×103 times, a range 2 from 10×103 times to 90×103 times, a range 3 from 100×103 times to 900×103 times, and a range 4 from 1,000×103 times to 9,000×103 times.
For measuring a correct scanning amount of charged particle beam in conventional charged particle beam equipment such as a scanning electron microscope and a focused ion beam processing apparatus, i.e., for measuring a correct magnification for an enlarged specimen image, a scanning secondary electron image or a scanning transmitted electron image of a microscale specimen (FIG. 4A) or crystal lattice fringe (FIG. 4B) has been used to measure an interval dimension of which indicates a dimensional feature.
The measured result, i.e., a deviation of an actually measured value from a reference dimension value is given as a magnification deviation, i.e., magnification error.
JP-A-2002-15691 describes a calibration of magnification for a scanning electron microscope using a standard specimen.